Ferromagnets are natural-born materials suitable for persistent, non-volatile recording media of information. In the present state of the art, ferromagnetic (FM) thin films with perpendicular magnetic anisotropy (PMA) may be widely employed as high-density data storage media. While a ferromagnet always may be controlled by a magnetic field, toward higher integration and lower energy consumption dictated by modern computing devices, it is highly desired to manipulate the magnetization via electrical means, instead of by a magnetic field. One implementation is a non-volatile magnetic random access memory (MRAM) utilizing spin-transfer torque (STT-MRAM). Such a device utilizes the spin-transfer torque (STT) generated by an electrical current to switch the magnetization of a free layer with respect to a magnetic pinning layer. This technique, however, requires high values of current density to pass through the magnetic layers, typically greater than 107 amperes-per-centimeters-squared (A/cm2), which is close to the breakdown limit of the device. A more reliable method to control the magnetization of a memory unit is desired for modern computing applications.
Another method has been proposed to overcome the aforementioned drawbacks of STT-MRAM. Instead of a spin-polarized charge current that carries STT, pure spin current may be exploited for manipulating the magnetic polarization, which is free of the damaging effects of a charge current (e.g., Joule heating and electromigration). A typical device using this technique may comprise a non-magnetic/ferromagnetic (FM) multilayer. The non-magnet layer may be a heavy metal (HM), such as platinum (Pt), tungsten (W), or alloys of Pt or W, which has strong spin-orbit coupling. A charge current flowing in the HM layer will subsequently generate a pure spin current which may inject into the adjacent FM layer, and may cause the magnetic moment of the latter to switch as if under an external torque. Therefore, the role of the spin current in such a process is usually termed as the spin-orbit torque (SOT). The advantage of this technique is that the charge current is not required to flow through the FM layer, which is potentially harmful to the magnetic memory unit. However, a large charge current of similar magnitude, 107 A/cm2, is still required to flow in the HM layer in order to generate SOT strong enough to switch the magnetization. Accordingly, there is a need in the art to generate SOT more efficiently and to replace the present design of HM/FM multilayer structure.